Is there a filter for get_post_custom()?

The quickest way to answer this question is to follow the code. See get_post_custom in Codex. Scroll down to Source Code section:

get_post_custom() is located in wp-includes/post.php

There we’ll see that get_post_custom calls get_post_meta, which then calls get_metadata, which points us to:

get_metadata() is located in wp-includes/meta.php

there we’ll finally see the available filter:

$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );

in this case, post is the $meta_type, so our filter is get_post_metadata